Computed tomography is an imaging technique that is used primarily for medical diagnostics and material analysis. In computed tomography, a radiation source and, acting in conjunction therewith, a radiation detector rotate around a scanned object to obtain spatially three-dimensional image data. During the rotational movement, scan data is acquired within an angular range. The scan data is a plurality of projections containing information about the attenuation of the radiation by the object. The scan data is also known as raw data. From this scan data, the image data can then be reconstructed, e.g. using so-called filtered back projection or an iterative reconstruction method. If the object moves during scanning, the motion may produce blurring and artifacts in the reconstruction of the image data.
Thus it is well known that cardiac motion during a CT scan can produce inconsistent and motion-artifact-impaired image data. As a result, the clinical usefulness of the image data is severely limited. To prevent motion artifacts, modern tomographic cardiac imaging uses phase-related acquisition of scan data by means of retrospective or prospective triggering. Thus in the case of prospective triggering, only data in a certain time interval around the resting phase of the heart cycle is acquired and used for image reconstruction. The common objective of these approaches is to “freeze” cardiac motion in order to reduce data inconsistency and therefore increase image quality.
An approach for increasing the image quality for a partially and cyclically moving object is known from the publication DE 10 2011 083 643 A1. There, a method for determining a motion field in an image domain of a CT image dataset is disclosed. A projection dataset of a CT system is determined for a predefined motion phase and a range of projection angles. In addition, the motion field is iteratively determined by minimizing a cost function of CT image datasets of a first image resolution using a first analytical reconstruction algorithm, the cost function being based on raw data or transformed into the image domain and the data sets being reconstructed in a motion-compensating manner using motion fields.